Marked spoiler for potential vibe harshing
spoiler
It might be a mix of a lot of things. The Palestinian Genocide, Ukrainian War, Chinese Expansion, actual probable chance of a second Trump Presidency, and a great many other issues are probably weighing heavy on the minds of net denizens. That and less sunlight than in the summer months.
Plus I've never seen any posts from a Foodporn or cute pets lemmy community, but I have seen crimes against the culinary arts and some posts about pets that have died.
People are just not feeling optimistic, and that leads to feelings of repression and anger.